1. Field of the Invention
This invention relates to mechanical drive systems using stepper motors and in particular to such a drive system having special advantages for use in ink jet printers of the type having a rotating imaging drum and in other applications where constant velocity is a requirement and positional error must be small and noncumulative.
2. Brief Description of the Prior Art
The imaging drum of an ink jet printer must be capable of operation at a constant speed with a high degree of positional accuracy. Servo systems with direct current motors have been used for applications of this kind but have several drawbacks. If a low-cost, low-resolution positional (angular) encoder is used, the system is subject to cumulative positional errors and is not ideal for applications where the position of the driven object must be closely controlled. In an ink jet printer where it is desirable to provide resolution of the order of 2000 dots per inch over an eleven inch circumference, the dc motor servo requires the use of expensive and complex feedback. Such systems have been proposed using an expensive, high resolution encoder wheel in connection with the imaging drum and providing a slave electronic control circuit to regulate the speed. A synchronous motor does not provide an acceptable solution because of the excessive cost resulting from the accuracy requirement.
The use of a permanent magnet stepper motor would eliminate the problems associated with the cumulative error of low-cost direct current motor systems, but introduces new problems resulting from the natural oscillation caused by pulsing of stepper motors. When the rotor of a stepper motor is positioned at a pole position, zero torque is produced on the rotor, the torque increasing to a maximum mid way between the poles. The result is speed variation during the succesive steps and oscillation around the pole positions. Micro-stepping will provide a partial solution, but the result is not satisfactory for precision applications without a large increase in cost. The problem of oscillation is particularly apparent in applications in which the load has a relatively large inertia as does the imaging drum in an ink jet printer.